Core catcher and boiling water nuclear plant using the same

ABSTRACT

According to an embodiment, a core catcher has: a main body including: a distributor arranged on a part of a base mat in the lower dry well, a basin arranged on the distributor, cooling channels arranged on a lower surface of the basin connected to the distributor and extending in radial directions, and a riser connected to the cooling channels and extending upward; a lid connected to an upper end of the riser and covering the main body; a cooling water injection pipe open, at one end, to the suppression pool, connected at another end to the distributor; and chimney pipes connected, at one end, to the riser, another end being located above the upper end of the riser and submerged and open in the pool water.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.15/616,423, filed Jun. 7, 2017, which is based upon and claims thebenefit of priority from Japanese Patent Application No. 2016-115241,filed Jun. 9, 2016; the entire content of which is incorporated hereinby reference.

FIELD

The embodiments of the present invention relate to a core catcher and aboiling water nuclear plant using the same.

BACKGROUND

The core catcher is safety equipment designed to cope with severeaccidents that may occur in the nuclear plant. Even if the molten corefalls through the bottom of the reactor pressure vessel onto the floorof the containment vessel of the nuclear reactor, the core catcherreceives the core debris (i.e., residues of the molten core), and keepscooling the containment vessel of the nuclear reactor, therebypreserving the safety of the containment vessel and limiting the releaseof radioactive substances. As the radioactive substances existing in thecore debris decay, they keep generating heat that amounts to about 1% ofthe nuclear reactor output power. Without cooling means, the core debrismay melt through the base mat concrete of the containment vessel, and agreat amount of radioactive substances may be released into theenvironment. To prevent such an event, it is planned that a core catcherhaving cooling channels should be installed in the boiling water reactor(BWR).

The core catcher of the European ABWR (EU-ABWR), for example, has radialcooling channels. In the cooling channels, the cooling water in theupper part of the core catcher is recirculated, efficiently removing thedecay heat generated in the debris. This recirculation of cooling wateris natural circulation, and does not require active pumps. Further, thecooling water can be circulated uniformly in the cooling channelsbecause the cooling channels extend in radial directions.

Taking the conventional ABWR and the conventional European ABWR(EU-ABWR) for example, the containment vessel and core catcher used inthe conventional boiling water reactor (BWR) will be outlined withreference to FIG. 9 to FIG. 16.

(ABWR Shown in FIG. 9)

FIG. 9 is an elevational, cross-sectional view of a containment vesselof a conventional ABWR. FIG. 10 is a plan view of the containment vesselof the conventional ABWR. As shown in FIG. 9, a core 1 is provided in areactor pressure vessel 2. The reactor pressure vessel 2 is provided inthe containment vessel 3. The containment vessel 3 is shaped like ahollow cylinder (see FIG. 10).

The interior of the containment vessel 3 is partitioned into a dry well4 and a wet well 5. The dry well 4 and the wet well 5 each constitutes apart of the containment vessel 3. The wet well 5 has a suppression pool6 in it. The suppression pool 6 has a normal water level 6 a of about 7m. The suppression pool 6 holds pool water in a large amount, about3,600 m³. Above the suppression pool 6, a wet-well gas phase 7 isprovided. The wet-well gas phase 7 is about 12.3 m high. The outer wallparts of the dry well 4 and the wet well 5 are integrated, forming thehollow cylindrical outer wall 3 a of the containment vessel 3. Theceiling part of the dry well 4 is a flat plate. This part is called atop slab 4 a of the dry well 4.

In the case of the ABWR, the containment vessel 3 is made of reinforcedconcrete. Therefore, the containment vessel 3 of the ABWR is called“reinforced-concrete containment vessel (RCCV).” To make the containmentvessel gastight, steel liners (not shown) are laid on the inner surfacesof the containment vessel. FIG. 9 and FIG. 10 show an example of anRCCV. As seen from FIG. 10, the outer wall 3 a of the containment vessel3 is shaped like a hollow cylinder. The bottom part of the RCCV isconstituted by a part 99 a of a base mat 99. The RCCV is made ofreinforced concrete. The base mat 99 constitutes the bottom part of thereactor building 100. It is proposed that the base mat 99 could be madeof steel concrete composite (SC composite) in the future.

As shown in FIG. 9, the reactor pressure vessel 2 is supported by ahollow cylindrical pedestal 61 through a vessel skirt 62 and a vesselsupport 63. The pedestal 61 is constituted by a hollow cylindricalsidewall (i.e., pedestal sidewall 61 a). The pedestal sidewall 61 a hasa thickness of, for example, 1.7 m. The pedestal sidewall 61 a is madeof concrete, and has inner and outer layers made of steel. The outerlayer made of steel is strong enough to support, almost by itself, theweight of the reactor pressure vessel 2. The bottom of the pedestalsidewall 61 a contacts the base mat 99, and is supported by the base mat99.

Below the reactor pressure vessel 2 and the vessel skirt 62 in the drywell 4, a space is formed, surrounded by the hollow cylindrical pedestalsidewall 61 a and the part 99 a of the base mat 99. This space is calledpedestal cavity 61 b. In the RCCV of the ABWR, the pedestal sidewall 61a constitutes a boundary wall between the wet well 5 and the dry well 4.Particularly, the space of the pedestal cavity 61 b is called lower drywell 4 b. The height from the floor of this lower dry well 4 b to thebottom of the reactor pressure vessel 2 is about 11.55 m. The upperspace in the dry well 4, excluding the lower dry well 4 b, is calledupper dry well 4 c.

(Lower Dry Well Part Shown in FIG. 11)

FIG. 11 is an enlarged view of the lower dry well (lower DW) 4 b andperipherals. On the bottom of the lower dry well 4 b, a concrete floor67 is provided, having a thickness of about 1.6 m. The concrete floor 67has sumps 68. The sumps 68 have a depth of about 1.3 m. The sumps 68 areconfigured to collect leakage water therein if the coolant leaks fromthe pipes or components connected to the reactor pressure vessel 2. Thewater levels in the sumps 68 are monitored in order to detect theleakage. Two sumps 68, a high conductivity waste sump 68 a and a lowconductivity waste sump 68 b, are provided (see FIG. 10), but only onesump is shown in FIG. 9 and FIG. 11. Each sump 68 has a corium shield(i.e., a lid for preventing inflow of debris; not illustrated), whichprevents the in-flow of core debris in case a severe accident occurs.Various types of corium shields have been devised, one of which isdisclosed in Japanese Patent Application Laid-Open Publication2015-190876, the entire content of which is incorporated by reference.

In the lower dry well 4 b, there are provided control rod drives 10 anda control rod drive handling equipment 11. The control rod drives 10 areconnected to the bottom of the reactor pressure vessel 2. The controlrod drive handling equipment 11 is arranged below the control rod drive10. About 205 control rod drives 10 are used in all. The control roddrive handling equipment 11 takes the control rod drives 10, one by one,from the reactor pressure vessel 2, rotates each control rod drive 10 toa horizontal position and moves up the same again, so that the controlrod drives 10 may be carried out of the containment vessel. The controlrod drive handling equipment 11 is therefore indispensable for themaintenance of the nuclear reactor. The control rod drive handlingequipment 11 can rotate, in its entirety, in the horizontal direction tobe positioned with respect to each of the all control rod drives 10.This is why the upper surface of the control rod drive handlingequipment 11 is also called a turntable 11 a.

The control rod drive handling equipment 11 has a height of about 4.6 m,and can hold the control rod drives 10 in it. On the turntable 11 aoperators may stand to perform maintenance work. Therefore, the lowerends of the control rod drives 10 are spaced from the turntable 11 a byabout 2.2 m. On the other hand, the lower end of the control rod drivehandling equipment 11 is spaced away from the concrete floor 67 by about10 cm only. Thus, a gap is scarcely provided between the concrete floor67 and the lower end of the control rod drive handling equipment 11. Thelower end of the control rod drive handling equipment 11 is about 1.7 mabove the upper end of the part 99 b of the base mat 99. The uppersurface of the concrete floor 67 is about 1.6 m above the upper end ofthe part 99 b of the base mat 99. No space is therefore available toarrange the core catcher, and the core catcher is not arranged there. Inthe conventional ABWR, the lower dry well 4 b holds the control roddrives 10 and the control rod drive handling equipment 11, and cannotaccommodate a core catcher.

It is proposed that the lower dry well 4 b should be used as a space forthe core catcher and the device (i.e., hopper) associated with the corecatcher (see, for example, Patent Application Laid-Open Publication2008-241657, the entire content of which is incorporated by reference).In practice, however, the lower dry well 4 b of the conventional ABWRhas no extra clearance, and a core catcher (disclosed in PatentApplication Laid-Open Publication 2008-241657) cannot be arranged there.

The size and shape of the containment vessel of the conventional ABWRare standardized as described above. The height from the upper end ofthe part 99 b of the base mat 99 to the lower end of the top slab 4 a(i.e., total height of the containment vessel 3) is about 29.5 m.

The dry well 4 and the suppression pool 6 are connected by LOCA ventpipes 8. Ten LOCA vent pipes 8, for example, are arranged (see FIG. 10),though only two LOCA vent pipes are shown in FIG. 9 and FIG. 11. Each ofthe LOCA vent pipes 8 has a plurality of horizontal vent pipes 8 asubmerged in the pool water and has openings in the pool water. In thecase of the RCCV, three horizontal vent pipes 8 a are provided for eachof the LOCA vent pipes 8 and extend in the vertical direction. Theuppermost horizontal vent pipe has its upper end located at the heightof about 3.85 m from the part 99 b of the base mat 99.

If an accident occurs, the suppression pool 6 is used as water sourcefor the safety system such as an emergency core cooling system. Even insuch a case, the pool keeps holding water in such an amount that thewater level never falls below the level of about 0.61 m to 1.0 m higherthan the upper end of the uppermost horizontal vent pipe 8 a. Thismeasure is taken in order that the horizontal vent pipe 8 a can keep acondensation function. Hence, in the event of an accident, the water inthe suppression pool 6 can be maintained at a level of about 4.46 m to4.85 m at the lowest.

In the RCCV, the LOCA vent pipes 8 are arranged, extending in theinterior of the pedestal sidewall 61 a shaped like a hollow cylinder.The pedestal sidewall 61 a is therefore called “vent wall 61 c” if usedin the case of the RCCV. As specified above, the vent wall 61 c is about1.7 m thick and made of concrete, and its inner and outer layers aremade of steel. The outer layer made of steel can support, by itself, theweight of the reactor pressure vessel 2. The concrete part of the ventwall 61 c reinforces the pedestal 61 and has the function of holding theLOCA vent pipes 8. The LOCA vent pipes 8 and the pedestal 61 constitutea part of the containment vessel 3.

One of the methods of maintaining, in the suppression pool 6, water muchenough to keep the water temperature low to cope with a severe accidentis to supply water to the pool from an external water source. Variousmeans (not shown) are available for supplying water to the suppressionpool, such as a portable pump, a fire-fighting pump and an alternatewater supply pump.

The design pressure of the containment vessel 3 is about 3.16 kg/cm²(0.310 MPa in terms of gauge pressure). The hollow cylindrical outerwall 3 a and the top slab 4 a are made of reinforced concrete and havethickness of about 2 m and a thickness of about 2.4 m, respectively.Their inner surfaces are lined with steel liners (not shown) for thepurpose of limiting the leakage of radioactive substances. The base mat99 has a thickness of about 5 m and is made of reinforced concrete, too.

The containment vessel 3 has a design leakage rate of about 0.4% perday. Recently it is proposed that the hollow cylindrical outer wall 3 aand the top slab 4 a of the containment vessel 3 could be made of steelconcrete composite (SC composite), not reinforced concrete. The SCcomposite comprises two steel frames secured to each other with ribs andconcrete filled in the gap between the steel frames. The SC composite isadvantageous in that rebars need not be laid and that it can bemodularly assembled. Further, as the SC composite is stronger, raisingthe design pressure of the containment vessel 3 even higher is possible.An example of employing an SC composite in nuclear plants is the shieldbuilding of the AP1000 (registered trademark) of Westinghouse, Inc.

(Eu-Abwr Shown in FIGS. 12 and 13)

How an EU-ABWR core catcher is installed will be explained withreference to FIG. 12 and FIG. 13. FIG. 13 is an enlarged view of thelower dry well 4 b.

As shown in FIG. 12 and FIG. 13, a core catcher 30 is mounted on thepart 99 b of the base mat 99 provided at the lower part of the lower drywell 4 b. The core catcher 30 is arranged eliminating the concrete floor67 (FIG. 9 and FIG. 11) about 1.6 m thick provided in the conventionalABWR. Further, in the EU-ABWR, the lower dry well 4 b is about 2.1 mhigher than in the ordinary ABWR, and the space for the core catcher 30has a height of about 3.7 m including the thickness of the eliminatedconcrete floor, i.e., 1.6 m. The core catcher 30 has height of about2.45 m.

Furthermore, a lid 31 is arranged above the core catcher 30. The upperend of the lid 31 lies about 3.6 m above the upper end of the base mat99. The lid 31 has a sump 68. The sump 68 is about 1.3 m deep. The lid31 is positioned so high that the sump 68 does not interfere with thecore catcher 30. The lower end of the control rod drive handlingequipment 11 is located about 3.7 m from the upper end of the base mat99. Hence, the core catcher 30 having a height of about 2.45 m can bearranged together with the lid 31 having a height of about 3.6 m.

(Fusible Valve)

In the pedestal cavity 61 b, fusible valves 64 and lower dry wellflooding pipes 65 are provided to cope with a core meltdown that mightoccur. The lower dry well flooding pipes 65 extend from the LOCA ventpipes 8, penetrate the pedestal sidewall 61 a and are connected to thefusible valve 64. One fusible valve 64 and one lower dry well floodingpipe 65 are provided on each of the LOCA vent pipes 8. Each fusiblevalve 64 has a plug part made of low-melting-point material, and opensby melting the plug part if the temperature in the lower dry well 4 brises to about 260 degrees centigrade.

If a core meltdown occurs, the corium melts through the bottom of thereactor pressure vessel 2, falls down into the pedestal cavity 61 b,melts through the control rod drive handling equipment 11, and is heldin the core catcher 30 provided at the bottom of the pedestal cavity 61b. Accordingly, as the temperature abruptly rises in the pedestal cavity61 b, the fusible valves 64 open. The cooling water in the LOCA ventpipes 8 then flows through the lower dry well flooding pipes 65 into thepedestal cavity 61 b, flooding and cooling the corium on the corecatcher 30. The cooled corium partly becomes solidified core debris. Thecooling water in the LOCA vent pipes 8 are supplied from the suppressionpool 6 through the horizontal vent pipes 8 a.

The configuration of the core catcher of the conventional EU-ABWR willbe described with reference to FIG. 14 to FIG. 16. FIG. 14 is asectional view showing the configuration of the core catcher of theconventional EU-ABWR. FIG. 15 is a plan view showing the configurationof the core catcher of the conventional EU-ABWR. FIG. 16 is aperspective view of one of the cooling channels used in the core catcherof the conventional EU-ABWR.

(Configuration of FIG. 14)

As shown in FIG. 14, the core catcher 30 is provided on the bottom ofthe lower dry well 4 b surrounded by the pedestal sidewall 61 a and thepart 99 b of the base mat 99. The core catcher 30 is constituted by adish-shaped basin 32. The basin 32 is made of steel and has a thicknessof about 1 cm. In some cases, the thickness of the basin 32 may be about5 cm, about 10 cm, or the like, depending on the strength the basin 32must have. A refractory layer 33 is laid on the basin 32, and asacrificial layer 34 is laid on the refractory layer 33. The refractorylayer 33 is composed of refractory bricks glued together, and has athickness of about 17.5 cm. The refractory bricks may be made of alumina(aluminum oxide) and zirconia (zirconium oxide).

The sacrificial layer 34 is made of concrete and has thickness of about5 cm. If core debris falls on to it, the sacrificial layer 34 is erodedwith the heat of the core debris, preventing the refractory layer 33from being heated over the allowable temperature, until the coolingwater is supplied from the fusible valve 64 and starts cooling the coredebris. The peripheral part of the basin 32 is connected to a circularannular riser sidewall 38 a having an axis extending in the verticaldirection. Around the riser sidewall 38 a, a circular annular downcomersidewall 39 a is provided and spaced from the riser sidewall 38 a byabout 10 cm. The upper edge of the downcomer sidewall 39 a lies at aheight of about 2.45 m from the upper end of the part 99 b of the basemat 99. The lid 31 is provided above the core catcher 30. The lid 31lies at a height of about 3.6 m above the part 99 b of the base mat 99.

The lid 31 is configured to fall onto the sacrificial layer 34immediately if the molten core falls from above. Thereafter, the lid 31melts due to the high temperature of the core debris and becomes part ofthe debris. Below the basin 32, many radial cooling channels 35 areprovided (see FIG. 15). The cooling channels 35 incline at about 10degrees. The cooling channels 35 have a length of about 4 m. The coolingchannels 35 are defined by many channel sidewalls (ribs) 35 a providedbelow the basin 32 (see FIG. 15 and FIG. 16). The number of coolingchannels 35 used is, for example, 16, and may be changed as needed. Thechannel sidewalls 35 a perform the function of cooling fins and ribssupporting the basin 32. The channel sidewalls 35 a are made of metalhaving high thermal conductivity, such as steel or copper.

A distributor 36 shaped like a hollow cylinder and having a verticalaxis is provided at the center of the radial cooling channels 35. Thediameter of the distributor 36 is, for example, about 2 m. The diameterof the distributor 36 may be changed if necessary. To the distributor36, the channel inlets 35 b of the cooling channels 35 are connected.The cooling water can therefore be uniformly supplied to all coolingchannels 35 from the distributor 36. The lower end of the distributor 36is closed by a bottom plate 36 a. The bottom plate 36 a contacts thepart 99 b of the base mat 99. In the distributor 36, a distributorpillar 36 b is provided as shown in FIG. 14. Alternatively, two or moredistributor pillars may be provided as needed. The distributor pillar 36b contacts the basin 32, whereby the distributor 36 bears a part of theweight of the basin 32.

The outlet ports of the radial cooling channels 35 are connected to ariser 38 that guides the cooling water upward in the vertical direction.The riser 38 is a flow passage provided between the riser sidewall 38 aand the downcomer sidewall 39 a, and has a width of about 10 cm. Theupper end of the riser 38, i.e., riser outlet 38 b, opens in the upperpart of the core catcher 30. The cooling water rises in the riser 38 andflows through the riser outlet 38 b into the upper part of the corecatcher 30.

Further, a circular annular downcomer 39 is provided, surrounding theriser 38. The downcomer 39 is a flow passage provided between thedowncomer sidewall 39 a and the pedestal sidewall 61 a and has a widthof about 30 cm. The upper end of the downcomer 39 opens in the upperpart of the core catcher 30. The downcomer 39 extends down to the bottomof the lower dry well 4 b and is connected to the cooling-water inletports 37 a of cooling water injection pipes 37. Each of the coolingwater injection pipes 37 has a cooling-water outlet port 37 b, which isconnected to the sidewall 36 c of the distributor 36.

In the configuration described above, the cooling water accumulated inthe upper part of the core catcher 30 flows down again in the downcomer39, reaches the distributor 36 through the cooling water injection pipes37, and is used in the cooling channels 35. Thus, the cooling water inthe upper part of the core catcher 30 is circulated again by thedowncomer 39. The basin 32, the cooling channels 35, the distributor 36and the cooling water injection pipes 37 are made watertight, and thecooling water would not leak from them.

If the fusible valves 64 are melted with the heat generated in the coredebris, the cooling water that floods the core debris existing above thebasin 32 and cools the core debris is supplied from the LOCA vent pipes8 through the lower dry well flooding pipes 65. Until the core debrisbecomes flooded by the cooling water, the sacrificial layer 34 protectsthe refractory layer 33 and the basin 32 from overheating, while thesacrificial layer 34 is melting.

The main body 30 a of the core catcher 30 is composed of the basin 32,the distributor 36, the cooling channels 35 and the riser 38. Therefractory layer 33 and the sacrificial layer 34 have the function ofprotecting the main body 30 a of the core catcher 30. The downcomer 39and the cooling water injection pipes 37 have the function ofcirculating the cooling water and supplying the cooling water to themain body 30 a.

(Configuration of FIG. 15)

FIG. 15 is a plan view of the core catcher used in the conventionalEU-ABWR, specifying the positions of the cooling channels 35 of the corecatcher. As shown in FIG. 15, the cooling channels 35 extend from thedistributor 36 in radial directions. The cooling channels 35 arepartitioned, one from another, by the channel sidewalls (ribs) 35 a.Each channel sidewall 35 a has an opening (not shown). In some case, thecooling water can flow from one cooling channel to another through theopening made in the channel sidewall 35 a. More channel sidewalls (ribs)35 a may be provided in order to strengthen the peripheral part of thecore catcher and to increase the number of heat transfer fins (see U.S.Pat. No. 8,358,732, the entire content of which is incorporated byreference).

(Configuration of FIG. 16)

FIG. 16 is a perspective view illustrating the configuration of thecooling channels 35 used in the core catcher of the conventionalEU-ABWR. In FIG. 16, the thicknesses of the walls are not shown.

Each cooling channel 35 is composed of a part 32 a of the basin 32, achannel sidewall 35 a, and a channel bottom wall 35 c, and is shapedlike a fan. The cooling channel 35 inclines, gradually upward to theouter circumference. The angle of inclination is about 10 degrees. Thechannel inlet 35 b of the cooling channel 35 is connected to thesidewall 36 c of the distributor 36. The other end of the coolingchannel 35 is connected to the riser 38. The riser 38 is composed of ariser sidewall 38 a, a riser rib 38 c, and a downcomer sidewall 39 a.

The cooling water flows into the cooling channels 35 through the channelinlets 35 b, is heated with the heat generated by the core debris, risesin the riser 38, and flows into the upper part of the core catcher 30through the riser outlet 38 b. Thereafter, again, the cooling waterflows down through the downcomer 39, then flows from the cooling-waterinlet port 37 a into the cooling water injection pipe 37, and furtherflows from the cooling-water outlet port 37 b into the distributor 36(see FIG. 14 and FIG. 15). The cooling water supplied into thedistributor 36 is circulated again in the cooling channels 35.

The drive force recirculating the cooling water results from the waterhead of the cooling water in the downcomer 39, which is about 2.45 mhigh. In order to acquire this drive force, the core catcher of theconventional EU-ABWR has a height of about 2.45 m except the lid 31. Thespace below the channel bottom wall 35 c is filled with concrete,embedding the cooling water injection pipe 37 therein. The channelbottom wall 35 c can thereby withstand the load applied to the coolingchannel 35. In some cases, a support member such as a rib may be used tosupport the channel bottom wall 35 c, instead of filling the space withconcrete.

(Disadvantages of the Prior Art)

In the containment vessel 3 of the EU-ABWR, the lower dry well 4 b has aheight about 2.1 m greater than the value used in the conventional ABWR.Hence, the levels of the reactor pressure vessel 2 and the core 1 areabout 2.1 m higher than the conventional ABWR. This reduces the seismicresistance. The reduction of seismic resistance is not so problematicin, for example, Europe where earthquakes are not severe, but should beavoided in a country, such as Japan, which suffers from severeearthquakes. Further, the total height of the containment vessel 3increases by about 2.1 m, and the total height of the reactor building100 increases by about 2.1 m, too. This increases the amount of concreteused and worsens the economy.

The containment vessel 3 of the EU-ABWR has a total height of about 31.6m, from the upper end of the part 99 b of the base mat 99 to the lowerend of the top slab 4 a. Accordingly, the core catcher 30 influences notonly the lower dry well 4 b, but also the entire plant including thecontainment vessel 3 and the reactor building 100.

One of the methods of avoiding such a problem is to dig down the part 99b of the base mat 99, i.e., bottom of the lower dry well 4 b, by about2.1 m and put the core catcher 30 therein. The rebars that have beenarranged in the conventional part 99 b of the base mat 99, that isdigged down, are to be cut and removed. In this case, however, theconfiguration of the base mat 99 becomes complicated, causing longerconstruction time and reducing the strength of the base mat 99 againstearthquakes.

In the containment vessel 3 of the conventional ABWR, the height of thelower dry well 4 b is not increased about 2.1 m, unlike in the EU-ABWR.In view of the construction schedule and the structure strength, it isundesirable to dig down the part 99 b of the base mat 99. Hence, if agap of about 10 cm is secured between the core catcher 30 and the lowerend of the control rod drive handling equipment 11, the height of thespace for accommodating the core catcher 30 is limited to about 1.6 m,because this space is provided by removing a part of the concrete floor67. As described above, the core catcher 30 of the EU-ABWR is about 2.45m high and the upper end of the lid 31 is about 3.6 m high.Consequently, the core catcher 30 cannot be disposed in the ABWR corecatcher space having a height of about 1.6 m.

The core catcher 30 can be disposed in the ABWR core catcher space about1.6 m high if the cooling channels 35 are inclined less, therebyreducing the thickness of the basin 32 and the height of the distributor36 is also reduced, and so on. In such a case, however, the height ofthe downcomer 39 decreases from about 2.45 m to about 1.6 m. The heightof the downcomer 39 determines the water head that is the drive forcefor circulating the water in the upper part of the core catcher 30 inthe cooling water injection pipes 37, the distributor 36, the coolingchannels 35 and the risers 38. Therefore, if the height of the downcomer39 decreases to about 1.6 m, the flow rate of the cooling water flowingin the radial cooling channels 35 inevitably decreases, and the decayheat generated in the core debris cannot be sufficiently removed.

The flow rate of recirculating the cooling water in the upper part ofthe core catcher 30 by the downcomer 39 is determined by the densitydifference between the cooling water in the radial cooling channels 35and the riser 38, and the cooling water in the downcomer 39. The lowerthe temperature of the cooling water in the upper part of the corecatcher 30 flowing into the downcomer 39, the larger the densitydifference will be. Generally speaking, however, the upper part of thecore catcher 30 holds the hottest core debris, which heats the coolingwater. It is therefore physically difficult to keep the cooling waterflowing into the downcomer 39 at low temperature. Accordingly, thecooling water in the upper part of the core catcher 30 is heated to ahigh temperature as time passes, though it remains at low temperatureimmediately after the core debris has fallen. Consequently, there is aproblem that the heated cooling water will impede a sufficient naturalcirculation flow rate.

Since the cooling water recirculated by the downcomer 39 contacts thecore debris existing in the upper part of the core catcher 30, part ofthe core debris may be released, flow into the downcomer 39 and move tothe lower part of the core catcher 30. If this happens, the core catcher30 would lose the function of holding and cooling the core debris. Toprevent this, a filter is arranged in the opening made in the downcomer39. The filter may be clogged with the loose parts scattered in theevent of a severe accident. The core debris may melt through the bottomof the reactor pressure vessel 2, and may then fall onto the upper partof the core catcher 30. Accordingly in this process, the thermalinsulators and such might become loose parts. Once the filter has beenclogged with the loose parts, the cooling water may not be recirculatedin a sufficient flow rate.

In the conventional core catcher 30, the cooling water does not exist inthe cooling channels 35 during the normal operation of the plant. If thecore debris falls, raising the temperature in the lower dry well 4 b andmelting the fusible valves 64, the water in the LOCA vent pipes 8submerges the core catcher 30 and the core debris, flows down in thedowncomer 39, passes through the cooling water injection pipe 37 anddistributor 36, and cools the cooling channels 35. Therefore, there is atime lag after the falling of the core debris until the cooling channels35 start cooling the basin 32. During this time lag, the sacrificiallayer 34 and the refractory layer 33 prevent the overheating of thebasin 32. However, if the impact of the falling core debris damages thesacrificial layer 34 and the refractory layer 33, the core debris maycontact the basin 32 directly and may melt a part of the basin 32.

An object of the present embodiments is to provide a thin core catcherwhich has a main body about 1.6 m or less high and can be arranged in alower dry well of a conventional ABWR without interfering with thecontrol rod drive handling equipment. Another object of the presentembodiments is to provide a core catcher which keeps cooling water inthe cooling channels during normal operation and enables coolingchannels to achieve cooling immediately if a sever accident occurred andcore debris fell onto it. Yet another object of the present embodimentsis to provide a thin core catcher which can, despite its smallthickness, preserve the flow rate of cooling water flowing in thecooling channels by means of natural circulation. Yet another object ofthe present embodiments is to provide a core catcher in which thecooling water on the upper surface is not recirculated in the coolingchannels, preventing the core debris and loose parts from flowing intothe cooling channels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-sectional view showing a situation wherean embodiment of a core catcher according to the present invention isarranged in a nuclear plant.

FIG. 2 is an elevational cross-sectional view showing a situation wherean embodiment of a core catcher is arranged in a lower dry well.

FIG. 3 is an elevational cross-sectional view showing a structure of afirst embodiment of a core catcher according to the present invention.

FIG. 4 is a plan view showing the structure of the first embodiment ofthe core catcher according to the present invention.

FIG. 5 is an elevational cross-sectional view showing a structure of amodification of the first embodiment of a core catcher according to thepresent invention.

FIG. 6 is an elevational cross-sectional view showing a structure ofanother modification of the first embodiment of a core catcher accordingto the present invention.

FIG. 7 is a plan view showing a structure of a second embodiment of acore catcher according to the present invention.

FIG. 8 is an elevational cross-sectional view showing the structure ofthe second embodiment of the core catcher according to the presentinvention.

FIG. 9 is an elevational cross-sectional view of a containment vessel ofa conventional ABWR.

FIG. 10 is a plan view of the containment vessel of the conventionalABWR.

FIG. 11 is an elevational cross-sectional view showing a structure of acontrol rod drive handling equipment and a concrete floor in acontainment vessel (lower dry well) of a conventional ABWR.

FIG. 12 is an elevational cross-sectional view showing a structure of acontainment vessel of a conventional EU-ABWR.

FIG. 13 is an elevational cross-sectional view showing a situation wherea control rod drive handling equipment and a core catcher are arrangedin the containment vessel (lower dry well) of the conventional EU-ABWR

FIG. 14 is an elevational cross-sectional view showing the structure ofthe core catcher of the conventional EU-ABWR.

FIG. 15 is a plan view showing the structure of the core catcher of theconventional EU-ABWR.

FIG. 16 is a perspective view of one of a cooling channel of the corecatcher of the conventional EU-ABWR.

DETAILED DESCRIPTION

According to an embodiment, there is presented a core catcher for use ina boiling water nuclear plant which has: a base mat; a reactor buildingbuilt on a part of the base mat; a containment vessel provided in thereactor building, built on the base mat and having a total height of notexceeding 29.5 m to a lower end of a top slab; a core; a reactorpressure vessel holding the core; a dry well constituting a part of thecontainment vessel and holding the reactor pressure vessel; a pedestalconnected to the base mat and supporting the reactor pressure vesselthrough a vessel skirt and a vessel support; a wet well constituting apart of the containment vessel, the wet well being provided around thepedestal, holding a suppression pool in a lower part thereof, and havinga wet well gas phase at an upper part thereof, LOCA vent pipes providedin a sidewall of the pedestal and connecting the dry well to thesuppression pool; a lower dry well which is a space in the dry well, islocated below the vessel skirt and the reactor pressure vessel and issurrounded by the sidewall of the hollow cylindrical pedestal and thepart of the base mat, which lies inside the sidewall of the pedestal;control rod drives provided in the lower dry well and connected to alower part of the reactor pressure vessel; and a control rod drivehandling equipment provided in the lower dry well and below the controlrod drives; the core catcher comprising: a main body including: adistributor arranged on the part of the base mat in the lower dry well,a basin arranged on the distributor, cooling channels arranged on alower surface of the basin, having inlets connected to the distributorand extending in radial directions, and a riser connected to outlets ofthe cooling channels and extending upward in vertical direction; a lidconnected to an upper end of the riser and covering the main body; acooling water injection pipe open, at one end, to the suppression pool,penetrating the sidewall of the pedestal, connected at another end tothe distributor, and configured to supply pool water to the distributor;and chimney pipes connected, at one end, to the riser, penetrating thesidewall of the pedestal, another end being located above the upper endof the riser and submerged and open in the pool water at a level lowerthan a minimum water level at a time of an accident, wherein the upperends of the main body and the lid are at heights lower than lower end ofthe control rod drive handling equipment, as measured from upper end ofthe base mat.

According to another embodiment, there is presented a boiling waternuclear power plant comprising: a base mat; a reactor building built ona part of the base mat; a containment vessel provided in the reactorbuilding, built on the base mat and having a total height of notexceeding 29.5 m to a lower end of a top slab; a core; a reactorpressure vessel holding the core; a dry well constituting a part of thecontainment vessel and holding the reactor pressure vessel; a pedestalconnected to the base mat and supporting the reactor pressure vesselthrough a vessel skirt and a vessel support; a wet well constituting apart of the containment vessel, the wet well being provided around thepedestal, holding a suppression pool in a lower part thereof, and havinga wet well gas phase at an upper part thereof; LOCA vent pipes providedin a sidewall of the pedestal and connecting the dry well to thesuppression pool; a lower dry well which is a space in the dry well, islocated below the vessel skirt and the reactor pressure vessel and issurrounded by the sidewall of the hollow cylindrical pedestal and thepart of the base mat, which lies inside the sidewall of the pedestal;control rod drives provided in the lower dry well and connected to alower part of the reactor pressure vessel; a control rod drive handlingequipment provided in the lower dry well and below the control roddrives; and a core catcher having: a main body including: a distributorarranged on the part of the base mat in the lower dry well, a basinarranged on the distributor, cooling channels arranged on a lowersurface of the basin, having inlets connected to the distributor andextending in radial directions, and a riser connected to outlets of thecooling channels and extending upward in vertical direction; a lidconnected to an upper end of the riser and covering the main body; acooling water injection pipe open, at one end, to the suppression pool,penetrating the sidewall of the pedestal, connected at another end tothe distributor, and configured to supply pool water to the distributor;and chimney pipes connected, at one end, to the riser, penetrating thesidewall of the pedestal, another end being located above the upper endof the riser and submerged and open in the pool water at a level lowerthan a minimum water level at a time of an accident, wherein the upperends of the main body and the lid are at heights lower than lower end ofthe control rod drive handling equipment, as measured from upper end ofthe base mat.

First Embodiment

A first embodiment of the present invention will be described withreference to FIG. 1 to FIG. 6. Any components identical to ones shown inFIG. 9 to FIG. 16 are identified by the same numbers in FIG. 1 to FIG.6, and will not be described repeatedly in the following description.

(Configuration of FIGS. 1 and 2)

FIG. 1 is a sectional view illustrating a situation where a core catcheraccording to the present invention is arranged in the containment vesselof an ordinary type ABWR. FIG. 2 is an enlarged view showing theposition the core catcher takes in the lower dry well 4 b of thecontainment vessel 3.

In FIG. 1 and FIG. 2, the main body 30 a of the core catcher 30 has aheight of no more than about 1.6 m. The main body 30 a of the corecatcher 30 is arranged in a space provided by removing that part of aconcrete floor 67 (see FIG. 11) having a height of about 1.6 m, at thebottom of the lower dry well 4 b of the ABWR containment vessel 3.Therefore, none of the heights of the lower dry well 4 b, thecontainment vessel 3 and the reactor building 100 are increased by about2.1 m, unlike in the EU-ABWR. The total height of the containment vessel3 is about 29.5 m, from the upper end of the part 99 b of the base mat99 to the lower end of the top slab 4 a.

The main body 30 a of the core catcher 30 is provided below a controlrod drive handling equipment 11 (about 1.7 m high), not contacting thelower end of the control rod drive handling equipment 11. A lid 31 isarranged also below the control rod drive handling equipment 11 (about1.7 m high), not contacting the lower end of the control rod drivehandling equipment 11. The upper end of the main body 30 a of the corecatcher 30 and the upper end of the lid 31 are below a height of 1.7 mfrom the upper end of the part 99 b of the base mat 99.

(Configuration of FIG. 3)

The first embodiment of the present invention will be described withreference to FIG. 3 to FIG. 6.

As shown in FIG. 3, the main body 30 a of the core catcher 30 accordingto this embodiment includes a basin 32, a distributor 36, coolingchannels 35 and a riser 38.

This embodiment differs from the prior-art apparatus in severalrespects. First, the cooling channels 35 are inclined by, for example, 5degrees (not 10 degrees as in the prior-art apparatus), and the mainbody 30 a of the core catcher 30 is thin, having the total height of nomore than about 1.6 m that is less than the height (1.7 m) of thecontrol rod drive handling equipment 11. Second, the lid 31 is provided,contacting the upper end of the main body 30 a of the core catcher 30(i.e., the upper end of the riser 38). Third, the lid 31 is providedwith no sumps. Fourth, no downcomers are provided. Fifth, the coolingwater injection pipe 37 penetrates the vent wall 61 c, and its distalend opens in the water in the suppression pool 6. Sixth, the upper endof the riser 38 is closed, not open to the upper part of the corecatcher 30. Seventh, a chimney pipe 40 is provided and connected at oneend to the riser 38. Eighth, the chimney pipe 40 penetrates the ventwall 61 c, and its distal end opens in the water in the suppression pool6. Finally, the chimney pipe 40 extends upward to a position higher thanthe riser 38.

The chimney pipe 40 has an opening 40 a in the suppression pool 6, at aheight which is higher than the height (i.e., about 2.45 m) of thedowncomer 39 of the core catcher 30 used in the conventional EU-ABWR andwhich is lower than the minimum water level (i.e., about 4.46 m to 4.85m) in the suppression pool 6 in the event of an accident. For example,the upper end of the chimney pipe 40 may lie at a height of about 4 m.The cooling channels may be identical in structure to those shown in,for example, FIG. 16.

FIG. 3 corresponds to a cross-sectional view of FIG. 4 taken along arrowC-C that runs through centers of the chimney pipes 40, but not centersof the LOCA vent pipes 8 (See FIG. 4). Chimney pipes 40 appear in FIG.3. LOCA vent pipes 8, however, do not appear in FIG. 3 because LOCA ventpipes 8 do not exist on the cross section taken along arrow C-C in FIG.4. For example, there are ten chimney pipes 40 in FIG. 4 in thisembodiment. Therefore, five pieces of arrow C-C can be drawn in FIG. 4crossing two pairing chimney pipes 40 although only one piece of arrowC-C is drawn for simplicity. FIG. 3 is identical to all the coolingchannels 35, cooling water injection pipes 37 and chimney pipes 40 alongall pieces of arrow C-C.

(Configuration of FIG. 4)

FIG. 4 is a plan view of a first embodiment of the core catcheraccording to the present invention. In FIG. 4, the cooling channels 35are shown as exposed, but none of the lid 31, the sacrificial layer 34made of concrete, the refractory layer 33 composed of refractory bricksand the basin 32 made of steel plate are not illustrated.

As shown in FIG. 4, the number of cooling channels 35 provided is, forexample, 10, and the number of LOCA vent pipes 8 used is, for example,10. The number of the cooling channels 35 is not limited to 10,nevertheless. If eight LOCA vent pipes 8 are used, 8, 16 or 32 coolingchannels 35 may be used in accordance with the cooling ability andstructural strength that are desired. The chimney pipes 40 arepositioned, not interfering with the LOCA vent pipes 8. In theconfiguration of FIG. 4, for example, each chimney pipe 40 is providedbetween two adjacent LOCA vent pipes 8. The chimney pipes 40 arearranged in the vent wall 61 c (see FIG. 3).

In the embodiment configured as described above, the cooling waterinjection pipes 37, the distributor 36, the cooling channels 35, theriser 38 and the chimney pipes 40 are kept communicated with thesuppression pool 6 at all times, and always filled with the pool waterof the suppression pool 6. During an accident, the pool water issupplied into the cooling channels 35 by virtue of the densitydifference between the water in the suppression pool 6 and the coolingwater flowing in the cooling channels 35, the riser 38 and the chimneypipes 40. The chimney pipes 40 have an opening 40 a at a height of about4 m. Therefore, in each chimney pipe 40 up to about 4 m, exists lowdensity cooling water heated to high temperature by the decay heat ofthe core debris. The water is vaporized, generating a two phase flow ineach chimney pipe 40 in some cases.

On the other hand, low-temperature, high-density water exists in thesuppression pool 6 to a height of about 4 m. By virtue of the densitydifference between the respective water, the cooling water can besupplied into the cooling water injection pipe 37. The water head in thesuppression pool 6 is about 4 m, much higher than the water head ofabout 2.45 m in the downcomer 39 of the conventional EU-ABWR corecatcher. Therefore, much larger natural circulation flow rate can beobtained. The suppression pool 6 contains a large amount of pool waterand can keep low temperature and high density of cooling water.Therefore, the large natural circulation flow rate can be maintainedowing to the large density difference.

A method for maintaining water at low temperature in the suppressionpool 6 for a long time in the event of a severe accident may be tosupply water from an external water source to the suppression pool 6, orto supply condensate from a passive containment cooling system to thesuppression pool 6 (refer to WO2016/002224, the entire content of whichis incorporated by reference).

In the conventional core catcher 30, the density difference decreasesbecause the downcomer 39 supplies the low-density, high-temperaturewater heated by the core debris above the basin 32. Consequently, it wasdifficult to keep a large flow rate of natural circulation. This problemcan be solved in this embodiment.

Further, the core catcher of the embodiment does not use forrecirculation the contaminated water existing above the basin 32 thatmight contain some core debris and loose parts. Hence, it is possible toeliminate the possibility of loss of cooling function due to theclogging of the cooling channels 35 and so on.

Furthermore, since the water is constantly supplied from the suppressionpool 6 into the cooling channels 35 during the normal operation, thecooling of the basin 32 can be immediately started in an accident, evenif the sacrificial layer 34 and the refractory layer 33 are damaged byan impact of core debris drop. Once the temperature of the basin 32rises, the cooling water existing in the cooling channels 35 before theaccident starts cooling the basin 32 naturally, and the cooling water isstably supplied thereafter by virtue of natural circulation.

Thanks to the above cooling mechanism, the sacrificial layer 34 and therefractory layer 33 may be eliminated in the core catcher of thisembodiment. The core debris existing above the core catcher 30 is cooledwith the cooling water supplied from the lower dry well flooding pipes65 through the fusible valves 64 that have been melted open (see FIG.14).

Since the chimney pipes 40 provide a water head of, for example, 4 m,the heights of the basin 32 and the riser 38 need not be increased. Themain body 30 a of the core catcher 30 can therefore be thin (or low inheight). Hence, it is possible to provide a core catcher that can bearranged in the space with about 1.6 m height at the bottom of the lowerdry well 4 b, where is the only available space for the conventionalABWR to install a core catcher.

Variations of the first embodiment of the present invention will bedescribed with reference to FIG. 5 and FIG. 6.

(Configuration of FIG. 5)

As shown in FIG. 5, the chimney pipes 40 penetrate the pedestal sidewall61 a, each extending upward and slantwise. So shaped, the chimney pipes40 have no elbow parts, reducing the flow resistance and increasing thenatural flow rate of the cooling water. Alternatively, the chimney pipes40 can have a smaller diameter for the same reason.

(Configuration of FIG. 6)

As shown in FIG. 6, the chimney pipes 40 penetrate the pedestal sidewall61 a in horizontal direction and then extend upward in the suppressionpool 6. So shaped, the chimney pipes 40 have less elbow parts thanotherwise. In addition, as they do not extend upward in the pedestalsidewall 61 a the chimney pipes 40 can be installed more easily.

Second Embodiment

A second embodiment of the core catcher according to the presentinvention will be described with reference to FIG. 7 and FIG. 8.

(Configuration of FIG. 7)

FIG. 7 is a plan view outlining the second embodiment of the corecatcher according to the present invention.

As shown in FIG. 7, two sumps 68, i.e., a high conductivity waste sump68 a and a low conductivity waste sump 68 b, are arranged. In thevicinity of the sumps 68, a lid 31, a basin 32, a refractory layer 33, asacrificial layer 34, cooling channels 35, channel sidewalls 35 a, ariser 38, and chimney pipes 40 are configured to avoid interference withthe sumps 68 a and 68 b and surround the peripheries of the sumps 68.The sumps 68 a and 68 b have a corium shield (not shown) each. In thisembodiment, the core catcher 30 can be arranged without interfering withthe sumps 68 a and 68 b.

(Configuration of FIG. 8)

FIG. 8 is an elevational sectional view outlining the second embodimentof the core catcher according to the present invention. FIG. 8corresponds to a cross-sectional view of FIG. 7 taken along arrows A-Aand B-B that run through centers of LOCA vent pipes 8 (See FIG. 7). LOCAvent pipes 8 appear in FIG. 8. Chimney pipes 40 and cooling waterinjection pipes 37, however, do not appear in FIG. 8 because they do notexist on the cross sections. Chimney pipes 40 and cooling waterinjection pipes 37 rather exist on the cross section taken along arrowC-C of FIG. 7 (See FIG. 7). A cross-sectional view of FIG. 7 taken alongarrow C-C is exactly the same as FIG. 3. For example, there are tenchimney pipes 40 in FIG. 7 in this embodiment. Therefore, five pieces ofarrow C-C can be drawn in FIG. 7 crossing two pairing chimney pipes 40although only one piece of arrow C-C is drawn for simplicity. FIG. 3 isidentical to all the cooling channels 35, cooling water injection pipes37 and chimney pipes 40 along all pieces of arrow C-C also in the secondembodiment (See FIG. 3).

A sump riser 38 d, a sump refractory layer 33 a, and a sump sacrificiallayer 34 a are arranged along the sidewall of the sump 68. The corecatcher 30 can therefore be arranged without interfering with the sumps68 a and 68 b.

Other Embodiments

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A core catcher for use in a boiling water nuclearplant which has: a base mat; a reactor building built on a part of thebase mat; a containment vessel provided in the reactor building, builton the base mat and having a total height of not exceeding 29.5 m to alower end of a top slab; a core; a reactor pressure vessel holding thecore; a dry well constituting a part of the containment vessel andholding the reactor pressure vessel; a pedestal connected to the basemat and supporting the reactor pressure vessel through a vessel skirtand a vessel support; a wet well constituting a part of the containmentvessel, the wet well being provided around the pedestal, holding asuppression pool in a lower part thereof, and having a wet well gasphase at an upper part thereof; LOCA vent pipes provided in a sidewallof the pedestal and connecting the dry well to the suppression pool; alower dry well which is a space in the dry well, is located below thevessel skirt and the reactor pressure vessel and is surrounded by thesidewall of the hollow cylindrical pedestal and the part of the basemat, which lies inside the sidewall of the pedestal; control rod drivesprovided in the lower dry well and connected to a lower part of thereactor pressure vessel; and a control rod drive handling equipmentprovided in the lower dry well and below the control rod drives; thecore catcher comprising: a main body including: a distributor arrangedon the part of the base mat in the lower dry well, a basin arranged onthe distributor, cooling channels arranged on a lower surface of thebasin, having inlets connected to the distributor and extending inradial directions, and a riser connected to outlets of the coolingchannels and extending upward in vertical direction; a lid connected toan upper end of the riser and covering the main body; a cooling waterinjection pipe connected at an inlet end to the suppression pool,penetrating the sidewall of the pedestal, connected at an outlet end tothe distributor, and configured to supply suppression pool water to thedistributor; and chimney pipes connected at an inlet end to the riser,penetrating the sidewall of the pedestal, and having an outlet endlocated above the upper end of the riser and submerged in thesuppression pool water at a level lower than a minimum water level at atime of an accident, wherein the upper ends of the main body and the lidare at heights lower than lower end of the control rod drive handlingequipment, as measured from upper end of the base mat, wherein thecooling water injection pipe, the distributor, the cooling channels, theriser and the chimney pipes are kept communicated with the suppressionpool at all times, and always filled with the suppression pool water,the core catcher further comprising: a sump; and a sump riser extendingupward along a sidewall of the sump, wherein the basin and parts of thecooling channels surround the peripheries of the sump in conformity witha shape of the sump.
 2. The core catcher according to claim 1, wherein arefractory layer is provided along an upper surface of the basin andalong a side of the riser.
 3. The core catcher according to claim 2,wherein a sacrificial layer is provided along the refractory layer. 4.The core catcher according to claim 1, wherein the upper ends of themain body and the lid are positioned lower than a level 1.7 m above theupper end of the base mat.
 5. The core catcher according to claim 4,wherein a refractory layer is provided along an upper surface of thebasin and along a side of the riser.
 6. The core catcher according toclaim 5, wherein a sacrificial layer is provided along the refractorylayer.